1. Field of the Invention
The present invention is generally related to the use of a global positioning system (GPS) and, more particularly, to a method for monitoring the operability of a GPS receiver in order to determine, or verify, the validity of the signals being received by the receiver from global positioning satellites.
2. Description of the Related Art
Since the initial conception of a global positioning system (GPS), with a number of satellites transmitting data for receipt by GPS receivers, many different improvements and applications have been developed. These improvements include numerous data handling and error reduction techniques and numerous strategies to increase the speed and reliability of the data transmission, reception, and handling used in the determination of a global position of a GPS receiver.
U.S. Pat. No. 5,451,964, which issued to Babu on Sep. 19, 1995, describes a method and system for resolving double difference GPS carrier phase integer ambiguity utilizing decentralized Kalman filters. A reference station is provided at precisely known coordinates and carrier phase data and pseudoranges for each received GPS satellites are calculated at both the reference station and the mobile GPS receiver. A communication link couples carrier phase data, pseudorange data and the known reference station position from the reference station to the mobile GPS receiver. A first Kalman filter within the mobile GPS receiver operates on GPS satellite ephemeris parameters and the calculated pseudoranges to obtain an error state vector for position and time. The GPS satellite ephemeris parameters and calculated carrier phase data are then applied to a second Kalman filter to obtain a predicted vector of velocity and clock rate error. Line of site range and range rates are then estimated utilizing the outputs of both Kalman filters and utilized, together with double difference phase measurements, to resolve the double difference carrier wave integer ambiguity providing an accurate set of coordinates for the mobile GPS receiver.
U.S. Pat. No. 5,736,962, which issued to Tendler on Apr. 7, 1998, describes the time since last fix annunciation system for GPS-based wireless rescue systems. It is used in a wireless emergency location system in which a cellular phone is utilized to communicate the location of an individual or vehicle by announcing position based on the output of a GPS receiver, time since last fix is computed and annunciated to permit emergency personnel to ascertain if the position information is valid. The velocity and direction of travel can also be annunciated for permitting emergency personnel to calculate the location of the phone and thus a certain individual.
U.S. Pat. No. 6,121,923, which issued to King on Sep. 19, 2000, describes a fixed site and satellite data-aided GPS signal acquisition method and system. It includes lengthening a pre-detection integration interval so that weak signals can be detected by operating coherently on captured signals. Coherency is insured by relying on the predictability of the satellite signal's data pattern. The described method significantly reduces correlation resolution time based on applying information regarding code phase delays from each satellite in view to reduce to code search space for other satellites being correlated.
U.S. Pat. No. 6,273,771, which issued to Buckley et al. on Aug. 14, 2001, discloses a control system for a marine vessel. A control system for a marine vessel incorporates a marine propulsion system that can be attached to a marine vessel and connected in signal communication with a serial communication bus and a controller. A plurality of input devices and output devices are also connected in signal communication with the communication bus and a bus access manager, such as a CAN Kingdom network, is connected in signal communication with the controller to regulate the incorporation of additional devices, such as a GPS receiver, to the plurality of devices in signal communication with the bus whereby the controller is connected in signal communication with each of the plurality of devices on the communication bus.
U.S. Pat. No. 6,407,701, which issued to Ito et al. on Jun. 18, 2002, describes a GPS receiver capable of calculating accurate 2DRMS. In the GPS receiver, GPS solutions are calculated by performing GPS measurements using a Kalman filter. The GPS receiver is provided with a computing system which calculates 2DRMS according to a predefined equation.
U.S. Pat. No. 6,485,341, which issued to Lanyi et al. on Nov. 26, 2002, discloses a method for controlling the average speed of a vehicle. The method is described with reference to selecting a desired average speed, measuring an actual speed, and maintaining a cumulative error determined as a function of the difference between the average speed and actual speed and the time over which the actual speed measurement was taken. Based on the cumulative total of speed-time error, a compensatory speed is calculated that will reduce the cumulative speed time-error to an acceptable tolerance range within a selected period of elapsed time.
U.S. Pat. No. 6,559,793, which issued to Eschenbach on May 6, 2003, describes a differential global positioning system using a coarse GPS data for a fast time to a precise first fix. A differential global positioning system (DGPS) using coarse data for the locations in space of GPS satellites for ranging to the GPS satellites and then determining a precise differentially corrected GPS location of a GPS user receiver is described. The coarse GPS data can be GPS almanac data or non-current GPS ephemeris data as long as the same data is used by both a GPS reference receiver and the GPS user receiver.
U.S. Pat. No. 6,727,848, which issued to Eschenbach on Apr. 27, 2004, describes a global positioning system using almanac data for a fast time to first fix. A system using coarse GPS orbital parameters for providing a precise GPS location is described. It includes a GPS reference receiver and a GPS user receiver. Coarse locations in space of GPS satellites are computed from almanac or old ephemeris data.
U.S. Pat. No. 6,775,615, which issued to Yule on Aug. 10, 2004, describes a method of obtaining a position fix in a GPS receiver and a GPS receiver for the same. The method comprises the steps of providing an estimate of clock offset between the GPS receiver and the GPS satellite, measuring a set of pseudoranges from the GPS receiver to GPS satellite, and either resolving the set of pseudoranges to obtain a position fix or providing an estimate of the position of the GPS receiver, resolving the set of pseudoranges to obtain a position fix correction, and obtaining a position fix using the estimate of the position of the GPS receiver and the position fix correction, wherein resolving the pseudoranges does not involve resolving the pseudoranges for either a new clock offset or clock offset correction.
U.S. Pat. No. 6,842,715, which issued to Gaal on Jan. 11, 2005, describes multiple measurements per position fix improvements. Methods and apparatus for improved position determination of a device using multiple pseudorange measurements for transmitting sources at known locations, such as GPS satellites, is described. A plurality of pseudorange measurements for each transmitting source is processed together to obtain a simplified maximum likelihood estimate for the pseudorange for that transmitting source of a common reference time.
U.S. Pat. No. 6,885,919, which issued to Wyant et al. on Apr. 26, 2005, discloses a method for controlling the operation of a marine vessel. A process is provided by which the operator of a marine vessel can invoke the operation of a computer program that investigates various alternatives that can improve the range of the marine vessel. The distance between the current location of the marine vessel and a desired waypoint is determined and compared to a range of the marine vessel which is determined as a function of available fuel, vessel speed, fuel usage rate, and engine speed. The computer program investigates the results that would be achieved, theoretically, from a change in engine speed. Both increases and decreases in engine speed are reviewed and additional theoretical ranges are calculated as a function of those new engine speeds. The operator of the marine vessel is informed when an advantageous change in engine speed is determined.
U.S. Pat. No. 7,267,068, which issued to Bradley et al. on Sep. 11, 2007, discloses a method for maneuvering a marine vessel in response to a manually operable control device. A marine vessel is maneuvered by independently rotating first and second marine propulsion devices about their respective steering axes in response to commands received from a manually operable control device, such as a joystick. The marine propulsion devices are aligned with their thrust vectors intersecting at a point on a centerline of the marine vessel and, when no rotational movement is commanded, at the center of gravity of the marine vessel. Internal combustion engines are provided to drive the marine propulsion devices. The steering axes of the two marine propulsion devices are generally vertical and parallel to each other. The two steering axes extend through a bottom surface of the hull of the marine vessel.
U.S. Pat. No. 7,305,928, which issued to Bradley et al. on Dec. 11, 2007, discloses a method for positioning a marine vessel. A vessel positioning system maneuvers a marine vessel in such a way that the vessel maintains its global position and heading in accordance with a desired position and heading selected by the operator of the marine vessel. When used in conjunction with a joystick, the operator of the marine vessel can place the system in a station keeping enabled mode and the system then maintains the desired position obtained upon the initial change in the joystick from an active mode to an inactive mode. In this way, the operator can selectively maneuver the marine vessel manually and, when the joystick is released, the vessel will maintain the position in which it was at the instant the operator stopped maneuvering it with the joystick.
U.S. Pat. No. 7,446,700, which issued to Yu et al. on Nov. 4, 2008, describes methods and apparatus for decreasing time to first fix of GPS receiver. It provides a GPS receiver which is capable of computing a local time reference without employing RTC circuitry and employing a nonvolatile storing unit to store backup navigation data, wherein the local time reference and the backup navigation data can be used to reduce the TTFF of the GPS receiver. The GPS receiver includes a nonvolatile storing unit and a positioning unit. The positioning unit is capable of retrieving the backup navigation data from the nonvolatile storing unit and computing a local time reference after the GPS receiver is powered up. The positioning unit is further capable of employing the backup navigation data and the local time reference to computing the position of the GPS.
The patents described above are hereby expressly incorporated by reference in the description of the present invention.
Under certain circumstances, a receiver can experience a loss of GPS fix. When that happens, information relating to the global position of the receiver, as calculated by the software in the microprocessor of the receiver, becomes untrustworthy. In many applications, a temporary loss of fix is not extremely serious and, if the fix is regained within a relatively short period of time, the loss of fix is sometimes not even noticed. However, in other applications, a loss of fix can be very serious. For example, when a vehicle (e.g. a marine vessel) is being automatically controlled as a function of its GPS position, a loss of fix can cause a dangerous condition. It would therefore be very beneficial if a method could be provided for quickly and reliably detecting a loss of GPS fix condition and reporting that status to an operator of the vehicle. In addition, it would be significantly beneficial if, upon the detection of a loss of GPS fix, an alarm condition could be generated to halt all potentially dangerous actions that might subsequently be taken based on invalid information calculated on the assumption that the receiver was receiving valid and reliable information when, in fact, it was not because of a loss of fix.